The 2004 Biophysical Discussions will be on the topic, "Probing membrane microdomains." The meeting will be held in Asilomar, Monterey, CA from Oct. 28-31, 2004. All speakers will be describing techniques, including theoretical methods that can be used, in principle, to investigate lipid and protein lateral heterogeneity in membranes--a topic of commanding interest with the advent of the notion of rafts. The putative rafts are of basic interest to biophysicists studying membrane lateral organization and to biologists who need to know the functional consequences of lateral organization. However, the raft field is at a technical impasse because there is no unambiguous structural correlate of the biochemically defined raft. Different methods give different answers. This is because the membrane is an ordered two-dimensional liquid for which we are still developing the appropriate means of investigation. This Discussion will therefore focus on the currently available techniques that can be used to characterize membrane lateral heterogeneity--their advantages and limitations. These techniques include fluorescence and electron microscopic methods, NMR, mass spec. and X-ray studies, as well computational studies. Applications will include those focused both on model membranes and cell membranes. The meeting will also consider newly available techniques and what types of methodology should be developed in the future. The format of the Biophysical Discussions is unique in its stress on discussion and prior preparation by the participants.